High Nitrate Loads Research Articles

Hysteresis response of carbon release and nitrate reduction in polymer denitrification systems

Polymer denitrification has received much attention in the field of advanced wastewater treatment. It can release carbon source stably during long-term operation, which can be used as electron donor for denitrification. However, the response of the polymer denitrification system to the transient changes of nitrate is not sufficiently disclosed yet. In this study, the response of a polymer denitrification system to nitrate was comprehensively investigated through a series of experiments. Therefore, real-time response and hysteresis response phenomena were identified. The time dependence of microorganisms in the system and the recovery of the hysteresis response were elucidated. The experimental results revealed distinct response patterns before and after the hysteresis tipping point. The denitrifying microorganisms, which showed a high adaptive capacity, exhibited a real-time response over a range of low nitrate concentration variations (∼20–30 mg/L). In contrast, microbial recovery is poor over a range of high nitrate concentration variation (∼35–40 mg/L), which is referred to as a hysteresis response. Finally, the hysteresis response mechanism was revealed by monitoring the recovery of denitrification enzymes, gene and microbial communities. The results showed that transient shocks of high nitrate loads affect microbial community structure stability, denitrifying enzyme activity and gene expression. Meanwhile, the abundance of Microbacterium associated with carbon release was reduced. The combination of these factors leads to a hysteresis response in denitrification and carbon release. This work contributes to a deeper understanding of the hysteresis behavior in polymer denitrification systems, offering critical insights for optimizing system performance and improving nitrogen removal efficiency.

Methane-driven microbial nitrous oxide production and reduction in denitrifying anaerobic methane oxidation cultures

Methane (CH4) and nitrous oxide (N2O) are two important greenhouse gases (GHGs), and it is very important to achieve the reduction of both gases in the case of increasingly serious climate change. The study confirms the simultaneous mitigation of potent GHGs N2O and CH4 in denitrifying anaerobic methane oxidation (DAMO) enrichment, supplementing the understanding that DAMO microorganisms do not produce and reduce N2O. We observed rapid accumulation and consumption of N2O in DAMO cultures subjected to high nitrate and nitrite loadings. Moreover, when N2O served as the sole electron acceptor, a significant correlation was found between the flux of N2O reduction and CH4 oxidation. Inhibition of the particulate methane monooxygenase (pMMO) led to a notable decrease of 95% in N2O reduction and 50% in CH4 oxidation. Metagenomic analysis revealed that Candidatus Methanoperedens and Candidatus Methylomirabilis were the dominant genera, with all functional genes related to denitrification and methane oxidation identified, including NO reductase (nor) of Ca. Methylomirabilis. Transcriptomic analysis further confirmed that the transcripts of nor were one order of magnitude higher than other functional genes. These results indicate that when the substrate NO2- surpasses the dismutation capacity of NO dismutase (nod), DAMO bacteria can alternatively induce nor to reduce NO to N2O. However, considering that N2O reductase (nosZ) is still missing in Ca. Methylomirabilis, the partner Methyloversatilis may be responsible for N2O reduction. Our results provide new insights into the link between GHG emissions in DAMO systems under anaerobic conditions.

Treated wastewater and weak removal mechanisms enhance nitrate pollution in metropolitan rivers

The focus of urban water environment renovation has shifted to high nitrate (NO3−) load. Nitrate input and nitrogen conversion are responsible for the continuous increase in nitrate levels in urban rivers. This study utilized nitrate stable isotopes (δ15N–NO3− and δ18O–NO3−) to investigate NO3− sources and transformation processes in Suzhou Creek, located in Shanghai. The results demonstrated that NO3− was the most common form of dissolved inorganic nitrogen (DIN), accounting for 66 ± 14% of total DIN with a mean value of 1.86 ± 0.85 mg L−1. The δ15N–NO3− and δ18O–NO3− values ranged from 5.72 to 12.42‰ (mean value: 8.38 ± 1.54‰) and −5.01 to 10.39‰ (mean value: 0.58 ± 1.76‰), respectively. Based on isotopic evidence, the river received a significant amount of nitrate through direct exogenous input and sewage ammonium nitrification, while nitrate removal (denitrification) was insignificant, resulting in nitrate accumulation. Analysis using the MixSIAR model revealed that treated wastewater (68.3 ± 9.7%), soil nitrogen (15.7 ± 4.8%) and nitrogen fertilizer (15.5 ± 4.9%) were the main sources of NO3− in rivers. Despite the fact that Shanghai's urban domestic sewage recovery rate has reached 92%, reducing nitrate concentrations in treated wastewater is crucial for addressing nitrogen pollution in urban rivers. Additional efforts are needed to upgrade urban sewage treatment during low flow periods and/or in the main stream, and to control non-point sources of nitrate, such as soil nitrogen and nitrogen fertilizer, during high flow periods and/or tributaries. This research provides insights into NO3− sources and transformations, and serves as a scientific basis for controlling NO3− in urban rivers.

Encapsulation of denitrifying bacteria for potential recirculating aquaculture system denitrification

To solve the nitrate accumulation phenomenon in recirculating aquaculture system (RAS). This study aimed to provide a sufficient organic carbon source, denitrifying bacteria, and a suitable microenvironment for denitrification in RAS. Denitrifying capsules were prepared using the encapsulation method, the morphology, microbial community and influencing factors for nitrogen removal efficiency of the denitrifying capsules in synthetic RAS wastewater were investigated. The denitrifying capsules had a porous, rough, and loose structure, with a diameter of approximately 6 mm and a density of approximately 1.05 g per cubic centimeter. The effect of a low ratio of solid-to-liquid (10:1) and temperature (15 °C) on nitrate removal was significant during the initial acclimation period (1-10d). After the enclosed denitrifying bacteria activity was recovered, the influence was not significant. At the end of the test, the nitrate removal rate could reach more than 80% even at a lower ratio of solid-to-liquid (10:1) or at 15 °C. The capsule exhibited a short adaptation time for diverse types of substrates and adapted well to the substrate concentration. Under low influent nitrate or nitrite loading, the removal rate reached 80% on the 6th day, which was earlier than with high influent nitrate loading. The denitrification rate experiment showed that the nitrate removal rate could reach 80.14% at 14 h and finally stabilize at approximately 81.89%. The capsule-supported denitrification process was a zero-order reaction and the denitrification rate of the capsule was calculated to be 109.0 mg N/L-capsule·h. The longtime evaluation indicated that until the semen litchi powder was exhausted, the capsule could retain a high nitrate removal rate. Functional enzyme analysis reveals the nitrogen conversion pathway in the capsule included denitrification, dissimilatory nitrate reduction and nitrogenfixation. The porous structure and good adaptability to environmental conditions of the denitrifying capsule would help to achieve denitrification and reduce the nitrate content on the present RAS, without increasing infrastructure investment.

In Situ Reconstruction of Metal Oxide Cathodes for Ammonium Generation from High-Strength Nitrate Wastewater: Elucidating the Role of the Substrate in the Performance of Co3O4-x.

In situ electrochemical reconstruction is important for transition metal oxides explored as electrocatalysts for electrochemical nitrate reduction reactions (ENRRs). Herein, we report substantial performance enhancement of ammonium generation on Co, Fe, Ni, Cu, Ti, and W oxide-based cathodes upon reconstruction. Among them, the performance of a freestanding ER-Co3O4-x/CF (Co3O4 grown on Co foil subjected to electrochemical reduction) cathode was superior to its unreconstructed counterpart and other cathodes; e.g., an ammonium yield of 0.46 mmol h-1 cm-2, an ammonium selectivity of 100%, and a Faradaic efficiency of 99.9% were attained at -1.3 V in a 1400 mg L-1 NO3--N solution. The reconstruction behaviors were found to vary with the underlying substrate. The inert carbon cloth only acted as a supporting matrix for immobilizing Co3O4, without appreciable electronic interactions between them. A combination of physicochemical characterizations and theoretical modeling provided compelling evidence that the CF-promoted self-reconstruction of Co3O4 induced the evolution of metallic Co and the creation of oxygen vacancies, which promoted and optimized interfacial nitrate adsorption and water dissociation, thus boosting the ENRR performance. The ER-Co3O4-x/CF cathode performed well over wide ranges of pH and applied current and at high nitrate loadings, ensuring its high efficacy in treating high-strength real wastewater.

Anthropogenic impacts on the nitrate pollution in an urban river: Insights from a combination of natural-abundance and paired isotopes

Urban rivers are often characterized by high nitrate (NO3−) loadings. High NO3− loadings cause water quality and ecological damages, which undermines the sustainable development of cities. To date, the drivers of these high NO3− loadings remain unclear. This study, for the first time, integrated natural-abundance isotopes (δ15 N/δ18O–NO3− and δD/δ18O–H2O) and 15N-pairing techniques to comprehensively reveal the anthropogenic impacts on the NO3− pollution in an urban river. Natural-abundance isotopes suggested that in both the wet and dry seasons, the NO3− was predominantly from the conservative mixing of different sources, and biological NO3− removal was minor. The 15N-pairing experiments supported the natural-abundance isotope data, quantitatively showing that in-soil nitrification was prevailing, while NO3− removal processes (denitrification, anammox, and dissimilatory NO3− reduction to ammonium) were weak. A Bayesian isotope-mixing model showed that soil sources (soil organic nitrogen and chemical fertilizer) dominated the NO3− in the upper reaches, while in the lower reaches, the impermeable riparian zone short-circuited the access of soils to the river. Here, the wastewater treatment plants became a significant source of NO3−. This study quantitatively revealed the drivers of high NO3− loadings in an urban river, and generated important clues for effective NO3− pollution control and remediation in urban rivers.

Potential causes of partial-denitrification (PD) granular sludge breakdown under high nitrate loading rates

The granule instability has been frequently reported during the operation of high loading rates. While, there no research was performed on the recently developed anoxic partial-denitrification (PD) granules, a novel pathway in producing nitrite from nitrate for anammox process. Herein, this work, for the first time, investigated the influence of nitrate loading rates on the instability of PD granules and identified the key causes. Two lab-scale sequencing batch reactors (SBRs) were operated with nitrate loading rates (NLR) increased from 0.48 to 3.84 kg N/m3/d (R1, 8 cycles/d), and 0.96 to 7.68 kg N/m3/d (R2, 16 cycles/d) by gradually elevating the influent nitrate concentration. Results showed that nitrite production rates increased with the NLRs, with a maximal value of 5.26 kg N/m3/d obtained. However, the compact regular PD granules were not stable and broke down when NLR was above 3.84 kg N/m3/d, which resulted in serious sludge washing out from SBR. The high NLRs led to the extracellular polymeric substances (EPS) transformation in terms of its composition and structure, which the protein content in the EPS and the tightly bound EPS (T-EPS) fraction was significantly decreased, this was supposed to be the major reason causing the breakdown of PD granules. Besides, it was found the PD granule in R2 was more deteriorated than that in R1 under the same high NLR, suggesting the short starvation (idle) times in SBR cycle was likely another reason impairing the stability of PD granules. Overall, this research provides useful information in development of granule-based PD systems and sheds light on achieving high-rate nitrite production in SBR with great stability.

Characterization of the facultative anaerobic Pseudomonas stutzeri strain HK13 to achieve efficient nitrate and nitrite removal

A new facultative anaerobic denitrifier was isolated from anoxic activated sludge and identified as Pseudomonas stutzeri strain HK13. Single-factor experiments indicated that the optimal conditions for nitrate removal were a C/N ratio of 8, an initial pH of 7–10, a shaking speed of 100 rpm, and a temperature of 30–40 °C. When 100 mg/L NO3--N and NO2--N were used separately as sole nitrogen sources, the strain exhibited efficient denitrification ability with total nitrogen (TN) removal efficiencies of 98.22% and 98.58%, respectively, in 15 h with the corresponding maximum removal rates of 20.03 and 21.77 mg/L/h. And there was no nitrite accumulation finally in the nitrate denitrification system. In addition, 97.42% total oxidized nitrogen (TON) was removed at 20 °C and 93.46% NO3--N was reduced at 45 °C, suggesting that strain HK13 had a wide range of temperature and pH to adapt to the environment. Moreover, strain HK13 exhibited a notable ability to tolerate high nitrate loading (initial NO3--N > 450 mg/L) with a TON removal amount of over 200 mg/L/day. These findings demonstrate that strain HK13 is effective for nitrogen removal in wastewaters containing high concentrations of nitrate and nitrite.

Geochemical evolution of groundwater under the influence of human activities： A case study in the southwest of Poyang Lake Basin

Groundwater quality continues to be threatened globally by anthropogenic activities including excessive use of chemical fertilizers and sewage discharge. However, few studies on the evolution of groundwater chemistry have systematically characterized human and natural effects, while which is of great importance for sustainable water resources management. In this study, hydrochemical data and multiple tracers (δ15N–NO3-, δ18O–NO3-, and 87Sr/86Sr) were employed to elucidate the geochemical evolution of groundwater in an intensive human activity area - the southwest of Poyang Lake Basin where is characterized by the highly complex hydrogeological environment with the diversity of aquifer types and lithology. Combining fuzzy c-means (FCM) clustering with natural background levels (NBLs) analyses, groundwater was divided into four groups with hydrological and statistical significance: groundwater in recharge zones (group 2-1 and group 3), transition zones (group 2-2) and discharge zones (group 1). Using the ions relations and 87Sr/86Sr, we found that the dissolved components of groundwater in recharge zones were controlled by natural processes (water-rock interactions and groundwater runoff conditions), and human activities have altered the natural geochemical evolution of groundwater in transition zones and discharge zones. Multi-tracer approach (δ15N–NO3-, δ18O–NO3-, and 87Sr/86Sr) were used to analyze the sources of nitrate in groundwater. The results showed that the high nitrate load in groundwater was predominately from sewage, and the groundwater quality in local areas was also influenced by NH4+ fertilizer and manure. In addition, nitrate in groundwater was controlled by land use types that groundwater flows through, depending on hydrogeological conditions and soil properties. Inverse geochemical models were developed and constrained to evaluate the relative contributions of natural and anthropogenic sources and to establish a conceptual model of groundwater geochemical evolution in the study area. This study provides a way to evaluate the relative contributions of natural and anthropogenic sources on groundwater quality and explore the geochemical evolution of groundwater in human activity areas.

Rural population health risks caused by nitrate contamination of locally grown vegetables

Objectives to assess the levels of health risks in the rural population associated with the consumption of local vegetables contaminated with nitrates.
 Material and methods. We established the concentration of nitrates in local vegetables, grown on farms and private subsidiary plots of agricultural areas of the region using a retrospective analysis of the statistics from the Office of Federal Service for Surveillance on Consumer Rights Protection and Human Well-being (Rospotrebnadzor) in the Saratov region and our own research results. The health risks in the rural population from exposure to nitrates in local vegetables were assessed according to the Risk Assessment Guidelines (R 2.1.10. 1920-04).
 Results. It was revealed that local vegetable products are widely used in the diet of the rural population of the agricultural areas of the Saratov region, amounting to 203.422.5 kg/year per adult resident. Significant differences were revealed in the nitrate contamination of vegetables grown by farmers and on private subsidiary plots of agricultural areas of the Saratov region.
 Discussion. The high nitrate load detected in vegetable products of farms was determined by a significant amount of fertilizers applied to the soil in the form of ammonium nitrate. On the contrary, the use of special biomass from plants of the legume family by workers of personal subsidiary farms for feeding the growing vegetables, made it possible to obtain vegetable products with a significantly lower quantitative content of nitrates.
 Conclusion. The assessment of the hazard coefficients of nitrates in vegetable products of the studied areas testified to the possible effect of the nitrate component on the health of the local population. The individual carcinogenic risk caused by nitrate contamination of local vegetables was assessed as low; the population carcinogenic risk value associated with the presence of nitrates in vegetables had the possibility to contribute from 0.01 to 1.3 additional cases of malignant neoplasms to the general level of oncological morbidity in the population of the surveyed territories.

Effective partial denitrification of biological effluent of landfill leachate for Anammox process: Start-up, influencing factors and stable operation

Partial denitrification combined with Anammox is a promising approach for simultaneous removal of ammonium and nitrate from wastewaters. In this study, the start-up, influencing factors and stable operation of partial denitrification for treating biological effluent from landfill leachate were investigated. High nitrate loads (3.85 kg N m−3 d−1) and short hydraulic retention time (0.66 h) were obtained in the partial denitrification process, yielding a suitable ratio of NO2−-N/NH4+-N in the effluent for downstream Anammox process. The study also revealed the importance of carbon sources, COD/NO3−-N ratio and salinity in the partial denitrification. Acetate-type carbon source, COD/NO3−-N ratio of about 3.0 and salinity lower than 1% favored high-efficient partial denitrification. The endogenous carbon sources from high-rate partial denitrification sludge contributed to low COD consumption in the process. During the partial denitrification, the dominant genus of Thauera was enriched, and shifted to Pseudomonas with the increase of organic removal rates.

Influence mechanism of filling ratio on solid-phase denitrification with polycaprolactone as biofilm carrier

In this study, three up-flow fixed-bed bioreactors were constructed with three different filling ratios (filling volume/effective volume: 30%, 60% and 90%) of polycaprolactone (PCL). Above 98% of nitrate removal efficiency was achieved with low accumulations of nitrite and ammonium for each filling ratio. Low filling ratio of PCL had extensive folds and pores that favored the attachment and growth of microorganisms; however, excessive biomass restrained nitrate specific reduction rate (NaSRR). The most dominant genera were Comamonas (0.80–57.64%), Stenotrophomonas (2.59–54.39%), Acidovorax (7.32–23.55%), Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium (0.30–19.74%) and Thermomonas (0.12–14.58%). Nitrate reductase (EC 1.7.99.4), nitrite reductase (EC 1.7.2.1) and nitric oxide reductase (EC 1.7.2.5) predicted by PICRUSt2 were abundant in high influent nitrate load (NaL). According to the analysis of carbon balance model, the utilization rate (η) of PCL showed a highly positive correlation with influent NaL, indicating reducing filling ratio or HRT might be an effective measure to save cost for nitrate removal.

Contemporary systematics of vadose zone nitrate capture by speleothem carbonate

The movement of nitrate through the vadose zone has major implications for environmental and human health. This issue is particularly prevalent in karst terrain where agricultural activity, thin soils and dual permeability compound the problem of high nitrate loading to the overlying ecosystem. However, a paucity of records which document vadose zone nitrate concentrations prior to the 21st century render legacy nitrate dynamics, source attribution and baseline conditions to be poorly parameterised. Speleothems growing within karst cave settings may provide an opportunity to obtain records of vadose zone nitrate contamination which extend throughout the anthropogenic era. Here, we use dual isotope analysis of δ15NNO3 and δ18ONO3 in a contemporary study at Cueva-cubío del Llanío, N. Spain, designed to examine the transformation of nitrate between surface to cave environment, taking account of biogeochemical transformation, karst hydrology and partitioning as controls on the delivery of nitrate to the speleothem record. Concentrations of nitrate within speleothem calcite are low (measured range of 0.05 mM to 0.37 mM) due to partitioning (DNO3) across the dripwater-calcite interface. Values of δ15NNO3 extracted from cave waters in Cueva-cubío del Llanío (range +2.0 to +7.0‰) are shown to be excellent indicators of nitrate source and demonstrate no fractionation during incorporation into speleothem carbonate (range of δ15NNO3 in speleothem carbonate +1.6‰ to +6.4‰). Values of δ18ONO3 contained within cave waters (range −2.5‰ to +6.0‰) and speleothem carbonate (range +12.3‰ to +32.3‰) reflect a mixed signal of source, biogeochemical processing and hydrological pathway, providing critical insight into the behaviour of the karst aquifer. Contemporary systematics at Cueva-cubío del Llanío therefore confirm speleothem carbonate contains an excellent record of vadose zone nitrate. Analysis of nitrate contained within speleothem carbonate from other regions confirms the ubiquitous nature of partitioning across the water‑carbonate interface and the use of speleothem nitrate isotopes for recording surface ecosystem processes and vadose zone behaviour. Application of these principles to dated speleothem records should provide critical timeseries of nitrate loading, enabling understanding and remediation against the presence of vadose zone legacy nitrate.

Strategies to Overcome Intermediate Accumulation During in situ Nitrate Remediation in Groundwater by Hydrogenotrophic Denitrification.

Bioremediation of polluted groundwater is one of the most difficult actions in environmental science. Nonetheless, the clean-up of nitrate polluted groundwater may become increasingly important as nitrate concentrations frequently exceed the EU drinking water limit of 50 mg L–1, largely due to intensification of agriculture and food production. Denitrifiers are natural catalysts that can reduce increasing nitrogen loading of aquatic ecosystems. Porous aquifers with high nitrate loading are largely electron donor limited and additionally, high dissolved oxygen concentrations are known to reduce the efficiency of denitrification. Therefore, denitrification lag times (time prior to commencement of microbial nitrate reduction) up to decades were determined for such groundwater systems. The stimulation of autotrophic denitrifiers by the injection of hydrogen into nitrate polluted regional groundwater systems may represent a promising remediation strategy for such environments. However, besides high costs other drawbacks, such as the transient or lasting accumulation of the cytotoxic intermediate nitrite or the formation of the potent greenhouse gas nitrous oxide, have been described. In this article, we detect causes of incomplete denitrification, which include environmental factors and physiological characteristics of the underlying bacteria and provide possible mitigation approaches.

Understanding the origins of herbicides metabolites in an agricultural watershed through their spatial and seasonal variations

The aim of this study is to understand the spatial and seasonal variations of persistent herbicides metabolites and to determine their origins in the Vilaine River watershed, Britany-France. Improving knowledge on herbicides metabolites sources and seasonality is important for drinking water resource management. Data were collected at 13 sampling stations during five sampling campaigns in 2016 and 2017. Relations between water quality parameters, herbicides and metabolites were analyzed using statistical methods. The influence of land use and wastewater treatment plants (WWTP) on streams water quality has been identified. Cluster Analysis revealed that two groups of sampling stations can be described as “urban” with stations downstream the urban area and as “agricultural” with stations located downstream of the watershed. Chloroacetamids metabolites have been associated together with nitrates and agricultural areas as could be expected. Thus, the drinking water treatment plant located in the estuary of the Vilaine River is exposed to high metolachlor ESA and nitrate loads all year long. Aminomethylphosphonic acid (AMPA) is associated to anthropogenic urban contamination and nutrient loads. AMPA has its major sources in both glyphosate and phosphonate detergents issued from WWTP. This can help to adapt surface water treatment process and water management policies concerning herbicides metabolites.

Nitrate Removal Across Ecogeomorphic Zones in Wax Lake Delta, Louisiana (USA)

AbstractHuman activities have increased nitrate export from rivers, degrading coastal water quality. At deltaic river mouths, the flow of water through wetlands increases nitrate removal, and the spatial organization of removal rates influences coastal water quality. To understand the spatial distribution of nitrate removal in a river‐dominated delta, we deployed 23 benthic chambers across ecogeomorphic zones with varying elevation, vegetation, and sediment properties in Wax Lake Delta (Louisiana, USA) in June 2018. Regression analyses indicate that normalized difference vegetation index is a useful predictor of summertime nitrate removal. Mass transfer velocity were approximately three times greater on a vegetated submerged levee (13 mm hr−1), where normalized difference vegetation index was greatest, compared to other locations (4.6 mm hr−1). Two methods were developed to upscale nitrate removal across the delta. The flooded‐delta method integrates spatially explicit potential removal rates across submerged portions of the delta and suggests that intermediate elevations on the delta—including submerged levees—are responsible for 70% of potential nitrate removal despite covering only 33% of the flooded area. The channel network method treats the delta as a network of river channels and suggests that although secondary channels are more efficient than primary channels at removing received nitrate, primary channels collectively contribute more to overall removal because they convey more of the total nitrate load. The two upscaling methods predict similar rates of nitrate removal, equivalent to less than 4% of nitrate entering the delta. To protect coastal waters against high nitrate loads, management policies should aim to reduce upstream nutrient loads.

Bioreactors for brine denitrification produced during polluted groundwater desalination in fertigation areas of SE Spain: batch assays for substrate selection.

Increasing knowledge of nitrate removal using denitrifying bioreactors has illustrated the usefulness of this management practice for treating discharge water from agricultural land uses. The objective of this study was to assess the viability of almond shell, chopped carob, olive bone, and citrus woodchip as carbon media for denitrification of brine with high nitrate load (EC ≈ 20 dS m-1, NO3--N concentration ≈ 65-80 mg NO3--N L-1) in bioreactors. To the authors' knowledge, this is the first test of denitrifying brine using organic wastes as the carbon substrate, and the first use of these carbon media for that purpose. Nitrate removal efficiency and efficiency:cost ratio were considered. The results indicated that the best removal efficiency and cheapest cost were provided by citrus woodchip (3.02 ± 0.15 mg NO3--N m-3 d-1) at a cost of ≈ 6€ m-3, followed by almond shell (1.54 ± 0.20 mg NO3--N m-3 d-1) at a cost of ≈ 19€ m-3. Chopped carob and olive bone showed negligible nitrate removal in the brine; chopped carob generated acidic leachate with extremely high dissolved organic carbon, and olive bone resulted in a highly saline leachate. Of the four media tested, the results of this study indicated that citrus woodchip was the most suitable media for denitrification of the brine.

The effects of hydrological extremes on denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and mineralization in a coastal lagoon

Hydrological extremes of unusually high or low river discharge may deeply affect the biogeochemistry of coastal lagoons, but the effects are poorly explored. In this study, microbial nitrogen processes were analyzed through intact core incubations and 15N-isotope addition at three sites in the eutrophic Sacca di Goro lagoon (Northern Adriatic Sea) both under high discharge (spring) and after prolonged low discharge (late-summer) of the main freshwater inputs.Under high discharge/nitrate load, denitrification was the leading process and there was no internal recycling. The site located at the mouth of the main freshwater input and characterized by low salinity exhibited the highest denitrification rate (up to 1150 ± 81 μmol N m-2 h-1), mostly sustained by nitrification stimulated by burrowing macrofauna. In contrast, we recorded high internal recycling under low discharge, when denitrification dropped at all sites due to low nitrate concentrations, reduced bioturbation and nitrification. The highest recycling was measured at the sites close to the sea entrance and characterized by high salinity and particularly at the clams cultivated area (up to 1003 ± 70 μmol N m-2 h-1). At this site, internal recycling was sustained by ammonification of biodeposits, bivalve excretion and dissimilatory nitrate reduction to ammonium (DNRA), which represented 30% of nitrate reduction.Flash floods and high nitrate loads may overwhelm the denitrification capacity of the lagoon due to the reduced residence time and to the saturation of microbial enzymatic activity, resulting in high transport of nitrate to the sea. Prolonged dry periods favor large internal recycling, due to a combination of high temperatures, low oxygen solubility and low bioturbation, which may prolong the extent of algal blooms with negative effects on lagoon biogeochemical services. We conclude that hydrological extremes, which are expected to become more frequent under climate change scenarios, strongly alter N cycling in coastal sediments.

Biological Co-treatment of H2S and reduction of CO2 to methane in an anoxic biological trickling filter upgrading biogas

This study investigated the feasibility of co-treating H2S and CO2 in a biological trickling filter (BTF) inoculated with hydrogenotrophic methanogens (HMs) and nitrate-reducing, sulfur-oxidizing bacteria. This was accomplished by introducing a pure culture of Thiobacillus denitrificans in a BTF that was successfully upgrading a biogas mimic (60:40 CH4:CO2) to >97% methane using an enriched HM consortium. Nitrate was fed as the electron acceptor to oxidize H2S. The results revealed that a severe competition for hydrogen’s electrons occurred between carbon dioxide and nitrate. Due to this competition, N:S loading rates of 16:1 were required to achieve >98% H2S removal, a ratio which is four times greater than the theoretical N:S ratio for complete sulfur oxidation. However, such high nitrate loading rates (>50 g N–NO3- m−3 h−1) had a negative impact on the BTF’s biogas upgrading performance. An electron balance illustrated the increasing diversion of H2 electrons towards nitrate reduction as nitrate loading increased. Overall, this study showed that simultaneous biogas upgrading and H2S removal in a single bioreactor is possible, but that achieving high yields for both reactions requires further research in process and culture optimization.

Characterizing the impacts of land use on nitrate load and water yield in an agricultural watershed in Atlantic Canada

Excessive nitrate loading from agricultural non-point source is threatening the health of receiving water bodies at the global scale. Quantifying the drivers/sources of water and nitrate flux in watersheds and relating them to spatial and temporal land uses is essential for developing effective mitigation strategies. This study investigated the impact of land use on water yield and nitrate loading to surface water in a typical agricultural watershed in Prince Edward Island (PEI), Canada. We used historical streamflow and water quality records to calibrate the comprehensive hydrological model Soil and Water Assessment Tool (SWAT), which was setup with detailed annual land use records. The SWAT model performed well in predicting both daily streamflow and nitrate load. Land use demonstrated little impact on water yield but affected nitrate load significantly. Annual nitrate load ranged from 5.6 to 44.4 kg N ha−1 yr−1 for forest and soybean, respectively. Potato rotated land contributed 84.5% of annual nitrate load to the watershed. Source of water yield demonstrated high variability between the growing season and non-growing season. About 90% of water yield was contributed by groundwater during growing season, while runoff contributed over 60% of water yield during the non-growing season. Groundwater was the dominant source of nitrate loading for both seasons. The watershed estuary faced the highest threats from subbasins in the south western area due to the high nitrate load and proximity to the watershed outlet. Results by the machine learning algorithm random Forest analysis indicated that the climatic variables of temperature and precipitation were the top two factors affecting water yield, with a combined relative importance of 61%. Land use was the dominant factor affecting nitrate load, the relative importance of land use alone was ~50%. The results of this study provided critical insights for watershed management in Atlantic Canada.